Carbon compression plug question

[tx_newbie]

Hi. Just picked up a used bike with carbon fork/carbon compression plug. That spring sandwiched between the stem cap/base plate--what is it for, exactly? It seems to just sit between the cap and plate--Is it absolutely necessary to use? What happens if I ride without using the spring (assuming the thread bolt is the appropriate length).

thanks

 

[tafi]

A picture might help. Your description is not the best. Are you talking about the steerer expander nut?